Spinal implant

ABSTRACT

An intervertebral disc prosthesis for installation in, a spinal column includes a first intervertebral plate for engaging one or both of the inferior vertebral endplate and the inferior ring apophysis of a superior vertebral body. A second intervertebral plate for engages one or both of the superior vertebral endplate and the superior ring apophysis of an inferior vertebral body. A removable insert core is located between and engages the intervertebral plates. A projection projects from one of the intervertebral plates toward the other intervertebral plate. The removable insert core at least partially surrounds the projection when installed. The removable insert core is removable from between the intervertebral plates and from around the projection while the intervertebral plates and projection remain installed.

CROSS-REFERENCE TO RELATED APPLICATIONS

Benefit of U.S. Provisional Patent Application Ser. No. 60/822,460 filedAug. 15, 2006, is hereby claimed and the disclosure incorporated hereinby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to spinal implants and, more particularlyto intervertebral disc prostheses.

2. Description of Related Art

The spinal column comprises a series of vertebrae stacked on top of eachother. There are typically seven cervical (neck), twelve thoracic(chest), and five lumbar (low back) segments. Each vertebra has acylindrical shaped vertebral body in the anterior portion of the spinewith an arch of bone to the posterior, which covers the neuralstructures. Each vertebral body includes superior and inferiorendplates, which are respectively surrounded by superior and inferiorbony rings, called ring apophyses. Between each vertebral body is anintervertebral disc, a cartilaginous cushion to help absorb impact anddampen compressive forces on the spine. To the posterior, the laminararch covers the neural structures of the spinal cord and nerves forprotection. At the junction of the arch and anterior vertebral body arearticulations to allow movement of the spine.

Various types of problems can affect the structure and function of thespinal column. These can be based on degenerative conditions of theintervertebral disc or the articulating joints, traumatic disruption ofthe disc, bone or ligaments supporting the spine, tumor or infection. Inaddition congenital or acquired deformities can cause abnormalangulation or slippage of the spine. Slippage (spondylolisthesis)anterior of one vertebral body on another can cause compression of thespinal cord or nerves. Patients who suffer from one of more of theseconditions often experience extreme and debilitating pain and cansustain permanent neurological damage if the conditions are not treatedappropriately.

One treatment for spinal diseases and injuries is the removal andreplacement of the intervertebral disc with a prosthetic device. Someintervertebral prosthetic devices provide a degree of pivotal androtational movement, while others promote fusion of adjacent vertebrae.It would be desirable to provide an intervertebral disc prosthesis thatinitially provides relative movement between adjacent vertebrae, andwhich can be modified after installation to provide fusion of theadjacent vertebrae.

BRIEF SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, provided is anintervertebral disc prosthesis for installation in a spinal column. Theprosthesis includes a first intervertebral plate for engaging one orboth of the inferior vertebral endplate and the inferior ring apophysisof a superior vertebral body. A second intervertebral plate engages oneor both of the superior vertebral endplate and the superior ringapophysis of an inferior vertebral body. A removable insert core islocated between and engages the intervertebral plates. A projectionprojects from one of the intervertebral plates toward the otherintervertebral plate. The removable insert core at least partiallysurrounds the projection when installed. The removable insert core isremovable from between the intervertebral plates and from around theprojection while the intervertebral plates and projection remaininstalled.

In accordance with another aspect of the present invention, provided isan intervertebral disc prosthesis for installation in a spinal column.The prosthesis includes a first intervertebral plate for engaging one orboth of the inferior vertebral endplate and the inferior ring apophysisof a superior vertebral body. The first intervertebral plate has anaperture for permitting bone growth from the superior vertebral bodythrough the first intervertebral plates and has a plurality of upwardlyprojecting teeth. A second intervertebral plate engages one or both ofthe superior vertebral endplate and the superior ring apophysis of aninferior vertebral body. The second intervertebral plate has anotheraperture for permitting bone growth from the inferior vertebral bodythrough the second intervertebral plate, and has a plurality ofdownwardly projecting teeth. A removable insert core is located betweenand engages the intervertebral plates. The removable insert core is aresilient core having a generally C-shaped upper and lower profile. Theremovable insert core permits relative movement between the firstintervertebral plate and the second intervertebral plate. A downwardprojection, having a rounded distal end, projects downward from thefirst intervertebral plate toward the second intervertebral plate. Thesecond intervertebral plate forms a recessed portion for receiving therounded distal end of the downward projection. The removable insert coreat least partially surrounds the downward projection when installed inthe prosthesis. The removable insert core is removable from between theintervertebral plates and from around the downward projection while theintervertebral plates and projection remain installed.

In accordance with another aspect of the present invention, provided isan intervertebral disc prosthesis for installation in a spinal column.The prosthesis includes a first intervertebral plate for engaging theendplate of a first vertebral body. A second intervertebral plateengages the endplate of a second vertebral body that is adjacent to thefirst vertebral body. A projection projects from one of saidintervertebral plates toward the other one of said intervertebralplates. A generally C-shaped removable insert core is located betweenthe intervertebral plates and partially surrounds the projection. Thegenerally C-shaped removable insert core is removable from between theintervertebral plates and from around the projection while theintervertebral plates and projection remain installed.

In accordance with another aspect of the present invention, provided isan intervertebral disc prosthesis for installation in a spinal column.The prosthesis includes a first intervertebral plate for engaging one orboth of the inferior vertebral endplate and the inferior ring apophysisof a superior vertebral body. A second intervertebral plate engages oneor both of the superior vertebral endplate and the superior ringapophysis of an inferior vertebral body. A removable insert core islocated between and engages the intervertebral plates. Upper retainingtabs project downward from peripheral portions of the firstintervertebral plate. Lower retaining tabs project upward fromperipheral portion's of the second intervertebral plate and correspondto the upper retaining tabs. The removable insert core is radiallypositioned along the spinal column between said retaining tabs. Theremovable insert core is removable from between the intervertebralplates and retaining tabs while the intervertebral plates remaininstalled.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an example embodiment of anintervertebral disc prosthesis;

FIG. 2 is an exploded perspective view of the intervertebral discprosthesis;

FIG. 3 is a front elevation view of the intervertebral disc prosthesis;

FIG. 4 is a side elevation view of the intervertebral disc prosthesis;

FIG. 5 is a rear elevation view of the intervertebral disc prosthesis;

FIG. 6 is a is a front exploded view of intervertebral disc prosthesis;

FIG. 7 a is a plan view of a first intervertebral plate and assembledremovable insert core;

FIG. 7 b is a plan view of a second intervertebral plate;

FIG. 8 shows the intervertebral disc prostheses installed in a spinalcolumn; and

FIG. 9 is a perspective view of an example embodiment of anintervertebral disc prosthesis;

FIG. 10 is a front elevation view of an example embodiment of anintervertebral disc prosthesis;

FIG. 11 is a front elevation view of an example embodiment of anintervertebral disc prosthesis; and

FIG. 12 is a perspective view of an example embodiment of anintervertebral disc prosthesis.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to spinal implants. The present inventionwill now be described with reference to the drawings, wherein likereference numerals are used to refer to like elements throughout. It isto be appreciated that the various drawings are not necessarily drawn toscale from one figure to another nor inside a given figure, and inparticular that the size of the components are arbitrarily drawn forfacilitating the understanding of the drawings. In the followingdescription, for purposes of explanation, numerous specific details areset forth in order to provide a thorough understanding of the presentinvention. It may be evident, however, that the present invention can bepracticed without these specific details. Additionally, otherembodiments of the invention are possible and the invention is capableof being practiced and carried out in ways other than as described. Theterminology and phraseology used in describing the invention is employedfor the purpose of promoting an understanding of the invention andshould not be taken as limiting.

Example embodiments of an intervertebral disc prosthesis 1 forinstallation in a spinal column are shown in FIGS. 1-11. Theintervertebral disc prosthesis 1 is designed to be inserted anteriorlybeen adjacent superior (upper) and inferior (lower) vertebral bodies, toreplace a removed disc.

The intervertebral disc prosthesis 1 includes a first intervertebralplate 11 and a second intervertebral plate 12. The first intervertebralplate 11 engages the superior vertebral body. More specifically, thefirst intervertebral plate 11 engages one or both of the inferiorvertebral endplate of the vertebral body and the inferior ring apophysisof the vertebral body. The second intervertebral plate 12 engages anadjacent, inferior vertebral body. More specifically, the secondintervertebral plate 12 engages one or both of the superior vertebralendplate of the adjacent vertebral body and the superior ring apophysisof the adjacent vertebral body.

The intervertebral plates 11, 12 can have a generally planar shape.Alternatively, the intervertebral plates 11, 12 can also have a curvedshape, to better match the curved end face of a vertebral body.

As can be seen FIG. 4, the intervertebral plates 11, 12 include aplurality of teeth 13 for anchoring the plates 11, 12 to theirrespective vertebral bodies. In an example embodiment, theintervertebral plates 11, 12 have serrations, which provide asaw-toothed side or front profile, and which allow the plates to diginto and thereby anchor to adjacent vertebral bodies. The plurality ofteeth 13 can also be in the form of a plurality of pointed spikes.

In an example embodiment, the intervertebral plates 11, 12 include aplurality of apertures 14 or recessed portions. The apertures 14 orrecessed portions permit bone growth from the engaged vertebral bodiesinto the intervertebral plates 11, 12. The intervertebral plates 11, 12can also be coated with a porous material, to permit bone growth intothe porous material from the engaged vertebral bodies. For example, theintervertebral plates 11, 12 can include a hydroxyapatite coating.

One of the intervertebral plates, for example, the first intervertebralplate 11 includes a projection 15, which projects toward the otherintervertebral plate. The projection 15 can have short, generallycylindrical main body that attaches to the intervertebral plate at aproximal end, and a rounded, knoblike distal end, as shown in thefigures. It is to be appreciated that the projection 15 can take otherforms, such as rod or tab, for example, and can be provided on either ofthe intervertebral plates 11, 12. The projection 15 helps ensure properaxial and radial positioning of the intervertebral plates 11, 12 withinthe spinal column. The intervertebral plate that lacks the projection 15can include a recessed portion or aperture 16 for receiving the distalend of the projection 15. The distal end of the projection 15 rests inthe aperture 16 and can pivot and rotate within the aperture 16, whichallows relative movement between the intervertebral plates 11, 12. Theprojection 15 also serves to position and retain a removable insert core17 between the intervertebral plates 11, 12. In an embodiment, theprojection 15 includes a spring (not shown) that permits axial movementbetween the intervertebral plates 11, 12 along the spinal column.

The kinematics of the spine can be described by a range of rotationaround an instant Axis of Rotation (IAR)/Helical Axis of Motion (HAM).The projection 15 and aperture 16 can be located along the instant axisof rotation to allow the prosthesis 1 to better replicate the motion ofa spinal disc. As can be seen in, for example, FIG. 2, the projection 15and aperture 16 can be located slightly toward the posterior of theprosthesis 1, in accordance with the instant axis of rotation. However,it is to be appreciated that the projection 15 and aperture 16 can belocated in other positions, such as toward an anterior portion orlateral portion of the prosthesis 1.

Example materials of construction for the intervertebral plates includemetals such as stainless steel, titanium alloys, andcobalt-chrome-molybdenum alloys, or a bio-inert polymer, for examplepolyetheretherketones, such as the PEEK-OPTIMA® product, which iscommercially available from Invibio, Ltd.

The removable insert core 17 has a generally C-shaped upper and lowerprofile and fits around the projection 15. The removable insert core 17can be removably attached to the projection 15 by elastic and frictionforces provided by the removable insert core 17. For example, theremovable insert core 17 can “hug” the projection and is retainedbetween the intervertebral plates 11, 12 by hugging the projection 15.The knoblike distal end of the projection 15 can further aid inpositioning and retaining the removable insert core 17. Additionally oralternatively, the removably insert core 17 can be secured to theprojection 15 or one or both intervertebral plates 11, 12 via afastener, such as a screw 18.

The removable insert core 17 is located between and operatively engagesthe intervertebral plates 11, 12. For example, relative movement betweenthe intervertebral plates 11, 12 can apply force to the removable insertcore 17, which resists such relative movement. The removable insert core17 can directly engage the intervertebral plates 11, 12, by directcontact with the plates, or can indirectly engage the plates through,for example, a spacer (not shown).

The removable insert core 17 can be constructed from a resilient,generally flexible material, or a rigid, generally inflexible material.A resilient removable insert core 17 will allow relative movementbetween the intervertebral plates 11, 12, However, such relativemovement can be constrained by the insert 17, based on the physicalproperties of the insert 17. An example material of construction for aresilient removable insert core 17 is an elastomeric material, such assilicon or polyethylene. The removable insert core 17 could also beconstructed from a polymer, such as the PEEK-OPTIMA® product which iscommercially available from Invibio, Ltd. A rigid removable insert core17 will prevent relative movement between the intervertebral plates 11,12, and will provide fusion between vertebrae. A rigid removable insertcore 17 can include one or more passageways, to allow bone growththrough the passageways. Example materials of construction for a rigidremovable insert core 17 include metals, such as stainless steel,titanium alloys, and cobalt-chrome-molybdenum alloys, ceramics, andpolymers, such as the PEEK-OPTIMA® product, which is commerciallyavailable from Invibio, Ltd.

The removable insert core 17 is removable from between theintervertebral plates 11, 12 and from around the projection 15. Theremovable insert core 17 is removable while the intervertebral plates11, 12 remain installed in a spinal column. In the embodiments of FIGS.1-9, the removable insert core 17 is removed by first withdrawing thescrew 18, then pulling the insert 17 away from the projection 15. Theprojection 15 maintains proper spacing between the intervertebral plates11, 12 while the removable insert core 17 is removed. Additionaltemporary bracing can be provided to prevent undesirable relativemovement between the intervertebral plates 11, 12 while the removableinsert core 17 is removed. A new removable insert core 17 with similaror different properties when compared to the removed insert can beinstalled between the intervertebral plates 11, 12. For example, aresilient removable insert core 17 can be removed from a previouslyinstalled disc prosthesis, and replaced with a rigid insert. Such achange will modify a flexible prosthesis so that it becomes afusion-type prosthesis.

FIG. 8 shows the intervertebral disc prosthesis 1 installed betweenadjacent superior 19 and inferior 20 vertebral bodies. The prosthesis isinstalled by first removing a diseased or otherwise damaged disc. Thenthe intervertebral plates 11, 12 are respectively pressed into thesuperior 19 and inferior 20 vertebral bodies. The teeth 13 on theintervertebral plates help secure the plates 11, 12 to the vertebralbodies, by digging into the vertebral endplates and/or the ringapophyses. A removable insert core 17 having, for example, a desiredrigidity or flexibility is chosen and is inserted between theintervertebral plates 11, 12 and around the projection 15. It is to beappreciated that a suitable removable insert core 17 can be selectedbased on additional criteria, such as durability, compatibility with theintervertebral plates 11, 12, ability to adhere to the intervertebralplates 11, 12 and or the projection 15, etc. As discussed above, theremovable insert core 17 has a generally C-shaped upper and lowerprofile. It may be desirable to temporarily stretch open or widen theC-shaped profile, to facilitate the placement of the removable insertcore 17 around the projection 15. A suitable tool can be used for thispurpose. After the removable insert core is inserted between theintervertebral plates 11, 12, the fastening device is applied to securethe removable insert core to the projection 15. Alternatively, theremovable insert core 17 can be secured directly to the intervertebralplates 11, 12 themselves. If desired, the installed removable insertcore 17 can be removed and replaced with another core, while theintervertebral plates 11, 12 remain attached to their respectivevertebral bodies.

Turning to FIG. 9, in an example embodiment, the intervertebral discprosthesis 1 includes means for selectively preventing relative movementbetween the intervertebral plates 11, 12. As discussed above, when aresilient removable insert core 17 is installed in the prosthesis 1,relative movement between the intervertebral plates 11, 12 can occur.One method of preventing such relative movement, for example, whenfusion is desired, is by replacing the resilient insert core with arigid insert core. Another method of preventing such relative movementis by activating a selectively deployable blocking mechanism, to blockrelative movement in certain directions between the intervertebral,plates 11, 12. For example, the blocking mechanism can includedeployable pivot arms 21 that block relative movement between theintervertebral plates 11, 12. The pivot arms 21 are selectivelydeployable, and can be activated when needed. In an embodiment, thepivot arms 21 are spring-biased in the deployed, generally vertical,position, and can be held in a retained, generally horizontal, position.When a trigger, such as a lever or switch, is activated, thespring-biased pivot arms 21 are released to the deployed position. Thedeployed pivot arms 21 prevent relative movement between theintervertebral plates. If desired, the pivot arms 21 can be rotated backto and locked in the retained position, for later redeployment ifdesired. It is to be appreciated that the pivot arms 21 can be locatedon either intervertebral plate 11, 12 and need not be spring-biased.

In an embodiment, the resilient insert core is constructed from amaterial such that the properties of the resilient insert core can bechanged via piezoelectric stimulation. For example, the resilient insertcore can be made rigid via piezoelectric stimulation. This mitigates theneed to remove and replace the insert core, should a change to afusion-type prosthesis be desired. Electronics can be provided withinthe prosthesis itself, which are activated from outside the body, toprovide the piezoelectric stimulation necessary to change the propertiesof the insert core.

Turning to FIG. 10 in an example embodiment, the intervertebral discprosthesis 1 includes a first removable insert core 22, a secondremovable insert core 23, and a stabilizer plate 24 located between thefirst and second removable insert cores 22, 22. The stabilizer plate 24can be a generally planar metallic plate. Together, the removable insertcores 22, 23 and the stabilizer plate 24 can form a stack having agenerally C-shaped, upper and lower profile, as discussed above withrespect to the removable insert core 17 of FIGS. 1-8. Any one or more ofthe first removable insert core 22, the second removable insert core 23,and the stabilizer plate 24 can be removed and replace, as desired. Thestabilizer plate 24 adds additional rigidity to the removable insertcores 22, 22.

Turning to FIG. 11, in an example embodiment, an intervertebral discprosthesis 31 includes a first intervertebral plate 32, a secondintervertebral plate 33, and a removable insert core 34 located betweenthe intervertebral plates 32, 33. In the embodiment of FIG. 11, theremovable insert core 34 need not be generally C-shaped, although it canbe generally C-shaped, or some other shape, such as a circular orelliptical cylinder. Further, the projection discussed above need not beincluded, although it can be included. The first intervertebral plate 32includes upper retaining tabs 35 that project downward from peripheralportions of the first intervertebral plate 32. The second intervertebralplate 33 includes corresponding lower retaining tabs 36 that projectupward from peripheral portions of the second intervertebral plate 33.The retaining tabs 35, 36 radially position and retain the removableinsert core 34 along the spinal column and between the intervertebralplates 32, 33. The intervertebral plates 32, 33 can each include onelarge retaining tab or a plurality of smaller retaining tabs.

In an embodiment, the lower retaining tabs 36 include a channel 37 forreceiving a distal portion of the upper retaining tab 35. The channel 37helps to limit relative movement between the intervertebral plates 32,33. Further, means for selectively preventing relative movement betweenthe intervertebral plates 32, 33 can be provided, such as the deployableblocking mechanism discussed above.

Like the above-discussed embodiments, the embodiment of FIG. 11 caninclude either a resilient or rigid removable insert core 34. A rigidremovable insert core 34 can include one or more passageways, to allowbone growth through the passageways. Further, the intervertebral plates32, 33 can include apertures for permitting bone growth from thevertebral bodies into and through the apertures.

In an example embodiment, the second intervertebral plate 12 includes aninwardly protecting raised lip 41 and the removable insert core 17includes a corresponding groove 42. When the removable insert core 17 ispressed into the second intervertebral plate 12, the inwardly projectingraised lip 41 locks into the groove 42, which locks the intervertebralplates 11, 12 and removable insert core together. The prosthesis caninitially be provided with a slightly oversized resilient removableinsert core, which rests on top of the raised lip 41 on the secondintervertebral plate 12. When it is desired to change to a fusion-typeprosthesis, the oversized resilient core can be removed and a rigidinsert core having the groove 42 can be installed. The lip and groovesystem, along with the rigid insert core, locks the prosthesis firmlytogether.

The embodiments described above can preferably be used to supportadjacent cervical vertebrae in the anterior region of the vertebrae.However, persons skilled in the art would recognize that the disclosedembodiments may be utilized to support adjoining thoracic and lumbarvertebrae in the anterior, lateral or posterior regions of thevertebrae. Further, the disclosed embodiments can be used to join otherpieces of bone in other parts of the body.

It should be evident that this disclosure is by way of example and thatvarious changes may be made by adding, modifying or eliminating detailswithout departing from the fair scope of the teaching contained in thisdisclosure. The invention is therefore not limited to particular detailsof this disclosure except to the extent that the following claims arenecessarily so limited.

1. An intervertebral disc prosthesis for installation in a spinalcolumn, comprising: a first intervertebral plate for engaging one orboth of the inferior vertebral endplate and the inferior ring apophysisof a superior vertebral body; a second intervertebral plate for engagingone or both of the superior vertebral endplate and the superior ringapophysis of an inferior vertebral body; a removable insert core locatedbetween and engaging the intervertebral plates; and a projection thatprojects from one of said intervertebral plates toward the other one ofsaid intervertebral plates, wherein the removable insert core at leastpartially surrounds the projection when installed, and wherein theremovable insert core is removable from between the intervertebralplates and from around the projection while the intervertebral platesand projection remain installed.
 2. The intervertebral disc prosthesisof claim 1, wherein the removable insert core as a generally C-shapedupper and lower profile.
 3. The intervertebral disc prosthesis of claim1, wherein the removable insert core directly engages the intervertebralplates.
 4. The intervertebral disc prosthesis of claim 1, wherein thefirst intervertebral plate includes a porous coating for engaging thesuperior vertebral body and permitting bone growth into the porouscoating from the superior vertebral body, and wherein the secondintervertebral plate includes another porous coating for engaging theinferior vertebral body and permitting bone growth into the anotherporous coating from the inferior vertebral body.
 5. The intervertebraldisc prosthesis of claim 1, wherein the projection projects downwardfrom the first intervertebral plate.
 6. The intervertebral discprostheses of claim 5, wherein the second intervertebral plate forms arecessed portion for receiving a distal portion of the projection. 7.The intervertebral disc prostheses of claim 5, wherein the secondintervertebral plate forms an aperture for receiving a distal portion ofthe projection.
 8. The intervertebral disc prosthesis of claim 1,wherein the removable insert core is a resilient core that permitsrelative movement between the first intervertebral plate and the secondintervertebral plate.
 9. The intervertebral disc prostheses of claim 8,further comprising another removable insert core, and a rigid stabilizerplate located between said removable insert cores.
 10. Theintervertebral disc prostheses of claim 8, further comprising means forselectively preventing said relative movement between the firstintervertebral plate and the second intervertebral plate.
 11. Theintervertebral disc prosthesis of claim 8, wherein the removable insertcore has a generally C-shaped upper and lower profile.
 12. Theintervertebral disc prosthesis of claim 1, wherein the firstintervertebral plate forms a plurality of apertures for permitting bonegrowth from the superior vertebral body through the first intervertebralplate, and wherein the second intervertebral plate forms a secondplurality of apertures for permitting bone growth from the inferiorvertebral body through the second intervertebral plate.
 13. Theintervertebral disc prosthesis of claim 12, wherein the firstintervertebral plate and the second intervertebral plate each includerespective vertebral engaging surfaces having saw-toothed side profiles.14. The intervertebral disc prosthesis of claim 12, wherein the firstintervertebral plate includes a plurality of upwardly projecting teeth,and wherein the second intervertebral plate includes a plurality ofdownwardly projecting teeth.
 15. The intervertebral disc prosthesis ofclaim 12, wherein the removable insert core has a generally C-shapedupper and lower profile and includes a passageway for permitting bonegrowth from one or both of the first vertebral body and the secondvertebral body into the passageway.
 16. An intervertebral discprosthesis for installation in a spinal column, comprising: a firstintervertebral plate for engaging one or both of the inferior vertebralendplate and the inferior ring apophysis of a superior vertebral body,the first intervertebral plate having an aperture for permitting bonegrowth from the superior vertebral body through the first intervertebralplate and having a plurality of upwardly protecting teeth; a secondintervertebral plate for engaging one or both of the superior vertebralendplate and the superior ring apophysis of an inferior vertebral body,the second intervertebral plate having another aperture for permittingbone growth from the inferior vertebral body through, the secondintervertebral plate and having a plurality of downwardly projectingteeth; a removable insert core located between and engaging theintervertebral plates, wherein the removable insert core has a generallyC-shaped upper and lower profile, and further wherein the removableinsert core is a resilient core that permits relative movement betweenthe first intervertebral plate and the second intervertebral plate; anda downward projection that projects downward from the firstintervertebral plate toward the second intervertebral plate, thedownward projection having a rounded distal end, wherein the secondintervertebral plate forms a recessed portion for receiving the roundeddistal end of the downward projection, wherein the removable insert coreat least partially surrounds the downward projection when installed inthe prosthesis, and wherein the removable insert core is removable frombetween the intervertebral plates and from around the downwardprojection while the intervertebral plates and projection remaininstalled.
 17. An intervertebral disc prosthesis for installation in aspinal column, comprising: a first intervertebral plate for engaging theendplate of a first vertebral body; a second intervertebral plate forengaging the endplate of a second vertebral body that is adjacent to thefirst vertebral body; a projection that projects from one of saidintervertebral plates toward the other one of said intervertebralplates; and a generally C-shaped removable insert core located betweenthe intervertebral plates, wherein the generally C-shaped removableinsert core partially surrounds the projection, and wherein thegenerally C-shaped removable insert core is removable from between theintervertebral plates and from around the projection while theintervertebral plates and projection remain installed.
 18. Theintervertebral disc prosthesis of claim 17, wherein the removable insertcore is a resilient core that permits relative movement between thefirst intervertebral plate and the second intervertebral plate.
 19. Theintervertebral disc prosthesis of claim 17, wherein the projectionprojects downward from the first intervertebral plate.
 20. Theintervertebral disc prostheses of claim 19, wherein the projectionincludes a rounded distal end and the second intervertebral plate formsan aperture into which the rounded distal end extends.
 21. Theintervertebral disc prosthesis of claim 20, wherein the generallyC-shaped removable insert core is a resilient core that permits relativemovement between the first intervertebral plate and the secondintervertebral plate.
 22. The intervertebral disc prosthesis of claim17, wherein the first intervertebral plate forms a plurality ofapertures for permitting bone growth from the first vertebral bodythrough the first intervertebral plate, and wherein the secondintervertebral plate forms a second plurality of apertures forpermitting bone growth from the second vertebral body through the secondintervertebral plate.
 23. The intervertebral disc prosthesis of claim22, wherein the generally C-shaped removable insert core includes apassageway for permitting bone growth from one or both of the firstvertebral body and the second vertebral body into the passageway.
 24. Anintervertebral disc prosthesis for installation in a spinal column,comprising: a first intervertebral plate for engaging one or both of theinferior vertebral endplate, and the interior ring apophysis of asuperior vertebral body: a second intervertebral plate for engaging oneor both of the superior vertebral endplate and the superior ringapophysis of an inferior vertebral body; a removable insert core locatedbetween and engaging the intervertebral plates; upper retaining tabsthat project downward from peripheral portions of the firstintervertebral plate; lower retaining tabs that project upward fromperipheral portions of the second intervertebral plate and thatcorrespond to the upper retaining tabs; wherein the removable insertcore is radially positioned along the spinal column between saidretaining tabs, and wherein the removable insert core is removable frombetween the intervertebral plates and retaining tabs while theintervertebral plates remain installed.
 25. The intervertebral discprosthesis of claim 24, wherein the first intervertebral plate forms aplurality of apertures for permitting bone growth from the superiorvertebral body through the first intervertebral plate, and wherein thesecond intervertebral plate forms a second plurality of apertures forpermitting bone growth from the inferior vertebral body through thesecond intervertebral plate.
 26. The intervertebral disc prosthesis ofclaim 24, wherein the removable insert core is a resilient core thatpermits relative movement between the first intervertebral plate and thesecond intervertebral plate.
 27. The intervertebral disc prosthesis ofclaim 26, wherein each lower retaining tab includes a channel forreceiving the corresponding upper retaining tab.